added HTML comment support

@marcpmichel Thanks for the pr!

When processing HTML, there are more cases should be noticed, which I am planning to do now. For example:

verbose mode should produce "\" not "/ [content] /"

Currently nested if is not supported. The following case will not work correctly:

<body>
<!-- #!if CONDITION -->
<script>
  // #!if foo === 1
  const a = 1;
  // #!elseif bar === 1
  const a = 2;
  // #!elseif bar === 2
  const a = 3;
  // #!else
  const a = 4;
  // #!endif
</script>
<!-- #!else -->
<script>
  const a = 5;
</script>
<!-- #!endif -->
</body>

Also I am considering improving the code base to better support language specific extension. I will keep you informed when it is done.

afterwind-io / preprocessor-loader

added HTML comment support #2